The best hitter on a team is the one who contributes the most to team runs scored. It could be wOBA or OPS+ or something similar. If you wanted a cumulative stat for the entire season, maybe wRC. If someone is an average fan who doesn't care about analyzing, that is fine. But if someone tells me that the best hitter should bat third, or primarily uses batting average to judge a hitter, then they will get an argument.